1. Field of the Invention
The present invention relates to a technique for transmitting a signal for positioning. More specifically, the present invention relates to system for transmitting synchronized signals for positioning.
2. Description of the Background Art
A method of positioning using a signal transmitted from a satellite has been known. By way of example, a GPS (Global Positioning System) signal transmitted from a GPS satellite is used for positioning. More specifically, an apparatus receiving GPS signals transmitted from three or more GPS satellites read data contained in each of the signals, executes a predetermined operation, and calculates positional information. Here, accuracy of time data contained in the data determines the accuracy of positional information, and therefore, a high-precision clock (for example, an atomic clock) is mounted on each GPS satellite. It is noted that the clocks must be synchronized with each other. Therefore, in order to maintain synchronization between each of the clocks mounted on GPS satellites, sometimes it becomes necessary to correct time of a clock on a satellite using time information of a master clock provided on the ground.
A system enabling positioning based on satellite signals, that is, a so-called satellite positioning system may include, in addition to GPS operated by the United States of America as mentioned above, Galileo of which operation is under consideration of the European Union, GLONASS (Global Navigation Satellite System) operated by the Russian Federation and the like.
In the foregoing, conventional art related to the present invention has been described based on general technical information known to the applicant. To the best of applicant's memory, the applicant does not have any information to be disclosed as prior art, before the filing of this application.
When positional information is to be calculated using radio signals as described above, the following problems arise. As it is necessary to mount a high-precision clock on each satellite or a ground-based transmitter as a pseudolite, the cost for building a system for transmitting positioning signals increases.